At present, development of a solid oxide fuel cell (SOFC) is in progress. As an electrolyte for this fuel cell, stabilized zirconia is commonly used. However, the stabilized zirconia has low ionic conductivity at a lower temperature, whereby it is used at a higher temperature of at least 1000° C. Therefore, expensive ceramics had to be used instead of metals as components of the fuel cell.
In order to solve this problem, in recent years, perovskite type electrolytes of LaGaO3 which can be used at a lower temperature as compared with stabilized zirconia, were developed. Among them, it is reported that LaSrGaMgO3 shows a good-performance (KHuang, R. S. Tichy, and J. B. Goodenough, J. Am. Ceram. Soc., 81,2565(1998), U.S. Pat. No. 6,004,688, JP-A-11-335164 and JP-A-11-665165).
However, in the preparation of this LaGaO3 type compound, Ga as a typical element is not likely to constitute a perovskite structure. Therefore, firing at a high temperature is required, and there is a problem that heterogeneous phases of impurities other than the desired composition are likely to remain. As the heterogeneous phases of impurities, LaSrGaO4 having a melting point of about 1400° C. which is lower than that of perovskite and having low oxygen ionic conductivity, and LaSrGa3O7 having a melting point of at least 1600° C. and having low oxygen ionic conductivity, are typical ones.
In a solid state reaction method, oxides, carbonates or hydroxides of the respective metals are mixed, as they are, with starting materials by pulverizing, followed by firing. Therefore, microscopic unevenness in the mixed state tends to occur, whereby heterogeneous phases of impurities tend to remain. In order to prepare perovskite having little heterogeneous phases of impurities, firing at a high temperature of at least 1500° C. has been required.
On the other hand, in a case where the prepared solid composite oxide powder is to be molded into an electrolyte or an electrode for a fuel cell, usually, the solid composite oxide powder is press-molded, and then sintered by heating to a temperature of from 1300° C. to 1600° C. to obtain a sintered body structure.
Accordingly, when the oxide powder prepared by means of the solid state reaction method, is press-molded and sintered, the heterogeneous phases of impurities contained in the oxide powder are fused, and pores of the sintered body are covered with the heterogeneous phases of impurities having low oxygen ionic conductivity, whereby it has been difficult to form a homogeneous electrolyte body.
As described above, according to the solid state reaction method which has been used as a common preparation method, the temperature becomes high during firing in a state where the desired composition, an intermediate and a starting material are mixed, whereby there may be a case where a part of the mixture is fused to remain in the final product as heterogeneous phases of impurities.
As a common process for a production to constitute a composition at a lower temperature, a method using nitrates or acetates is known. However, in this method, a large amount of a harmful gas such as nitric acid gas, nitrogen oxide or acetic acid will be generated during the firing, and it is not suitable for the industrial production. Further, a sol-gel process using an organic solvent such as ethylene glycol as a solvent is also known, but a gel state substance is likely to deposit on a wall of a container and will burn intensely, and therefore this is not suitable for the industrial production either.
As a preparation method for other metal composite oxides, a citric acid method for an yttrium-alkaline earth metal-transition metal composite oxide, a bismuth-alkaline earth metal-transition metal composite oxide, a lanthanum-strontium-cobalt composite oxide or a lanthanum-cobalt-iron composite oxide, is disclosed in e.g. JP-B-7-96443, JP-3081212, JP-A-9-086928 or JP-A-08-130018, as proposed by the present inventors. However, each obtained oxide has low electrical conductivity in a low temperature range of from 600° C. to 800° C., and it is not suitable for a material for a solid oxide fuel cell of lower temperature operation type.
A solid oxide ceramics for a fuel cell is required to be a solid oxide of lower temperature operation type having particularly little heterogeneous phases of impurities. Therefore, the object of the present invention is to provide a composite oxide for a high performance solid oxide fuel cell of lower temperature operation type, which can be fired at a relatively low temperature, and which has little heterogeneous phases of impurities other than the desired composition, and an industrially advantageous process for its production.